It's sometimes because of motivation, but mostly it's due to injury.
For example, this time around, I maintained my fitness during the winter. I ran on the treadmill occasionally, swam and went on the bike. When it got warmed outside, I even did a few suicide runs, where my boyfriend would drive me 8-10km out and I'd run back home.
And then my knee/s started hurting. And it kept getting worse, until I went to see a doctor about it. He identified that I had tendon pain, and I took some time off and did a bunch of electro-therapy. Then I got better!
At the same time the knee was getting better, I decided to buy Vibrams. I thought the natural running pattern of the lighter shoes would force my stride to become better and would make everything better.
But I didn't factor in the time it should take to transition, and my calf got injured. (Mostly normal muscle tightness from being pushed too hard after it hadn't been run on - ever.)
So my calf hurt, and I took about 15 'rest' days where I biked and walked instead :)
Yesterday I had an amazing bike/run workout. I biked for an hour (15mi) and then went on the treadmill, with my Mizuno's. I thought I would try run for a minute, walk for a minute and repeat until I wanted to stop. But I ran for 3 minutes... nothing. I ran for two more minutes... I told myself I'd run for ten minutes and I would stop if I felt the knee twinge. I made it to a mile (11:11min) and NOTHING HURT. And of course, I wanted to keep running because it felt amazing. But I didn't want to push it, to jinx it, because I had a race at the end of the week.
So I guess what I'm trying to say is that whenever I feel great, and enjoy running, I jinx it by signing up for a race. And while 'training' for that race, I destroy my enjoyment of running. On top of staying healthy, there is the added pressure of having to run a race at the end of the training cycle. So after my second half marathon.... I'm not going to sign up for another for a while.
I want to run whenever I want, for however long I want to. I want to enjoy running, I want to stay injury-free, and I want to have lots and lots of races in my future. But I have to get back to running strongly and consistently, and that hasn't happened since before I started racing.
So no more signing up for races........... (unless I'm able to run injury-free through the summer).... or at least, not months in advance! ;)